The present invention relates to a device for preheating the two coil electrodes of a fluorescent lamp comprising an AC voltage source, a resonance capacitor, which is arranged serially between the two coil electrodes, and a first inductance, which is coupled between the AC voltage source and one of the two coil electrodes. It additionally relates to a method for preheating the two coil electrodes of a fluorescent lamp.
In order to explain the problems on which the invention is based, FIG. 1 illustrates a preheating device of the generic type, known from the prior art. An AC voltage source 10 is formed from a DC voltage source U, two switches S1, S2 and a coupling capacitor CK. The two coils W1, W2 of a fluorescent lamp La, in particular a low-pressure fluorescent lamp, are connected in series into the output circuit of an electronic ballast (not illustrated) which comprises an inductance L1 and also a resonance capacitor CR. This circuit provides a very simple method of providing heating of the coils W1, W2 before the ignition of the lamp La.
In the case of this circuit, preheating current IV and preheating voltage UV are related to the inductance L1 and the resonance capacitor CR as follows:             L1              C        R              -            U      v              I      v      
Accordingly, the voltage Uv across the lamp La during preheating and the preheating current Iv cannot be chosen independently of one another given predetermined values for the inductance L1 and the resonance capacitor CR. In the case of unfavorable lamp data or a lack of degrees of freedom in the design of the electrical ballast, it is possible, therefore, that adequate coil preheating will not be able to be obtained at the maximum permissible voltage Uv across the lamp La.
The object of the present invention therefore consists in enabling preheating which suffices for the ignition of the lamp even in the case of unfavorable lamp data or a lack of degrees of freedom in the design of the electrical ballast.
This object is achieved by virtue of the fact that the device of the generic type furthermore comprises a second inductance, which is connected to a point between the AC voltage source and the coil electrode coupled thereto, the second inductance being coupled to the first inductance in such a way that a current flow through the second inductance leads to a reduction of the magnetization of the first inductance.
This measure makes it possible to realize intensified coil preheating which manages with a minimum of additional components. The current through the first inductance L1, the so-called lamp inductor, is increased by connecting the second inductance, which is designed in particular as an auxiliary winding fitted on the lamp inductor, in a polarity such that the current flowing therein reduces the magnetization of the first inductance L1, that is to say effectively reduces the effective inductance. In accordance with the above formula, a reduction of the effective inductance corresponds to an increase in the preheating current. After preheating, the second inductance is switched out.
By virtue of the present invention, the fixed coupling between the lamp inductor L1 and the resonance capacitor CR is abandoned, so that a higher preheating current can be realized during preheating, on account of the smaller effective inductance, without the permissible maximum lamp voltage being exceeded.
In accordance with a second aspect of the present invention, the above object is also achieved by means of a method for preheating the two coil electrodes of a fluorescent lamp, using a preheating device having an AC voltage source, a resonance capacitor, which is arranged serially between the two coil electrodes, a first inductance, which is coupled between the AC voltage source and one of the two coil electrodes, and a second inductance, which is connected to a point between the AC voltage source and the coil electrode coupled thereto, which comprises the following steps:
firstly, the second inductance is coupled to the first inductance in such a way that a current flow through the second inductance leads to a reduction of the magnetization of the first inductance. Afterward, a current flow through the second inductance is effected during preheating and a current flow through the second inductance is prevented or the coupling between the first and second inductances is interrupted after preheating.
A particularly preferred embodiment comprises a control device by which the current flow through the second inductance or the coupling between the first and second inductances can be controlled. This control device is preferably realized as a switch arranged in series with he second inductance, or a PTC thermistor. For the case where the control device is realized as a switch, it is possible to provide a drive device which closes the switch during preheating and otherwise opens it. For the case where the control device is realized as a PTC thermistor, this driving is unnecessary since said PTC thermistor automatically undergoes transition to the high-impedance state at a specific temperature and thereby prevents a current flow through the second inductance.
The other connection of the second inductance is preferably connected to ground.